1. Field of the Invention
This invention relates to a low-carbon rail steel having a tensile strength of at least 900 N/mm.sup.2 and a yield point of at least 650 N/mm.sup.2. More particularly, this invention relates to a low-carbon steel rail which steel composition is substantially free of chromium whereby the rail itself has improved tenacity or plasticity and is not damaged by wear at low temperatures by the use of high velocity traffic.
2. Discussion of the Prior Art
Low-carbon rail steel having heretofore been provided as evidenced by U.S. Pat. No. 3,290,183. Therein there is disclosed the formation of a low-carbon rail steel containing 0.05 to 0.25 weight percent carbon, 0.1 to 1% molybdenum and particularly 2 to 6% chromium. The steel can also contain, in an alloy form, up to 1% silicon, up to 1.5% manganese, up to 1% nickel, up to 0.5% vanadium, up to 1% copper, up to 0.5% niobium, up to 0.5% titanium and up to 0.001% boron.
Unfortunately, this rail steel becomes quite brittle during the manufacture thereof when the same is cooled down from a hot rolling temperature. As this rail steel becomes too brittle during the air cooling it has to be cooled down from a temperature in the range of 500.degree.-750.degree. C to a temperature of 150.degree.-200.degree. C over a prolonged period of time which consumes more than 7 hours. The resultant rail steel has good tensile strength and can withstand weather and meets many other technological requirements for rails such as abrasion resistance, fatigue resistance and the like. The physical properties of the steel are due to its low carbon content coupled with its high chromium content and to the prolonged air cooling employed in its manufacture.
Inasmuch as prolonged and delayed air cooling lasting more than 7 hours noticeably disturbes the continuous production of steel rails it has become an object of the present invention to develop a rail without an expensive or extensive heat treatment which rail is made of a steel and has a tensile strength of at least 900 N/mm.sup.2, preferably at least 110 N/mm.sup.2 and a yield point of at least 650 N/mm.sup.2.
It is also known to produce non-heat treated rails having some of the properties mentioned. Thus steels having good tensile strength have been provided employing elements known to improve tensile strength. Steels have been provided having a carbon content of 0.37 to 0.82 weight percent, a silicon content of under 0.80%, and mangenese content of 0.60 to 2.10 percent and a chromium content under 1.7%. Such steels can also contain alloying elements such as molybdenum, vanadium, nickel and titanium. Such non-heated steels have a pearlitic structure and attain tensile strengths of up to 1100 N/mm.sup.2. These rails have a substantially favorable abrasion characteristic. See German Offenlengungsschrift 1 239 110.
Apart from abrasion resistance and fatigue strength characteristics a prime concern of rail steels is their plasticity and their resistance to rupture. Here consideration must be given to the fact that rails comprise a base, web and a top and have a complicated profile with clear differences in cross-section and shape. Thus damage in colder weather is often found which can be due to an unfavorable tenacity in steel. This damage is of the type which is worsened by high velocity traffic on the steel rail.
It therefore became an object of this invention to provide an improved steel rail which has excellent tensile strength, abrasion resistance and fatigue resistance and which had the desired tenacity whereby it would not be damaged in colder weather even under conditions of high velocity traffic. It also became desirable to provide such a steel which did not depend upon a high chromium content and/or an expensive or extensive heat treatment and/or cooling and, more especially, it became desirable to provide a steel of the type above described having a particularly high resistance against rupture and good plasticity (tenacity).